The invention relates to a UV light stabilization additive package for various applications. More particularly, the invention relates to a UV light stabilization additive package for use in an encapsulant material for solar cell module and laminated glass applications.
Transparent encapsulant materials are used in numerous applications, including solar cell module and laminated glass applications. In solar cell applications, transparent encapsulants protect and seal the underlying solar cells without adversely affecting the optical properties of such underlying materials. In laminated glass applications, transparent encapsulants minimize any possible hazards from broken glass. In these applications, the encapsulant is exposed to the ultraviolet (UV) rays of the sun and this exposure can result in the yellowing and physical degradation of the polymer. To prevent this, UV stabilizers are added to the encapsulant.
In the manufacture of crystalline silicon solar cell modules, a transparent encapsulant material is used to protect the brittle silicon solar cells from breakage and to help seal these cells into the overall module structure. The encapsulant material is usually a thermoplastic. The thermoplastic is melted, then flows to fill in any open spaces in the module and bonds to all adjacent surfaces. The most widely used encapsulant material for solar cell modules is a co-polymer of vinyl acetate and ethylene, known as ethylene vinyl acetate (EVA). EVA is used to encapsulate and seal both thin film and crystalline silicon solar cell modules.
There are several disadvantages associated with using EVA as an encapsulant material that adversely affect the quality and manufacturing cost of the solar cell modules. First, an organic peroxide is added to EVA in order to cross-link it using the heat which accompanies the lamination process. The cross-linking is necessary to increase the creep resistance of the encapsulated structure. However, the peroxide is not completely consumed during the cross-linking process, and the remaining peroxide can promote subsequent oxidation and degradation of EVA. In addition, the EVA must be laminated in a vacuum when making a module because of the presence of peroxide in the EVA. The reason for this is that oxygen lowers the degree of cross-linking, producing an unsatisfactory encapsulant. Second, the preferred EVA usually contains 33% (by weight) of vinyl acetate, and thus is a very soft and tacky substance that tends to stick to itself. This tackiness makes handling of the EVA material in a manufacturing environment much more troublesome. As such, the EVA material requires a release paper or liner material to use the material. Third, peroxide cured EVA has been known to turn yellow and brown under extensive exposure to sunlight for several years. Yellowing and browning causes reduction in solar module power output. Fourth, EVA can produce acetic acid under processing conditions which can then foster metal contact corrosion. Fifth, EVA is known to be fairly permeable to water and is, therefore, far from ideal as a sealant.
Although virtually any transparent polymer eventually shows some degradation and yellowing after exposure to sunlight, an encapsulant material that can withstand degradation and yellowing for a longer period of time than EVA is desirable. Ideally, a solar cell module should last for thirty years without showing much sign of degradation. EVA is unlikely to satisfy this thirty year duration requirement. In addition to finding a suitable replacement for EVA (or PVE, which is described below), it is also necessary to develop a suitable UV light stabilization package for the encapsulant.
In laminated glass applications, the laminated glass is made by forming a sandwich of two pieces of glass with a sheet of a transparent polymer disposed between the two pieces. This transparent polymer sheet serves to prevent the glass in the laminated structure from shattering into dangerous shards when the glass is broken. Windshields on automobiles and architectural glass are manufactured in this manner. Poly vinyl butyral (PVB) is a widely used material in such polymer sheets in the foregoing laminated glass applications. PVB, however, has several drawbacks. First, PVB is extremely hydroscopic (i.e. it absorbs moisture readily). Therefore, it must be kept refrigerated and maintained under special atmospheric conditions before it can be successfully laminated. Second, PVB is also extremely soft and tacky and, therefore, must be used with a release or liner layers.
This invention features an ultraviolet (UV) light stabilization additive package for use in an encapsulant material, which may be used in solar cell modules, laminated glass and a variety of other applications. The UV light stabilization additive package includes a first hindered amine light stabilizer and a second hindered amine light stabilizer. The first hindered amine light stabilizer provides thermal oxidative stabilization, while the second hindered amine light stabilizer provides photo-oxidative stabilization.
One aspect of the invention is that neither an ultraviolet absorber (UVA) nor an anti-oxidant (AO) is needed to provide the UV light stabilization. Anti-oxidants are used to insure that the polymer does not oxidize excessively and thereby suffer degradation of its properties, while it is being formed into a sheet or a film. Experiments on an encapsulant material including the UV stabilization additive package indicate that the encapsulant material does not suffer any loss in physical properties with repeated extrusions even without the addition of an anti-oxidant. The stabilization additive package of the present invention does not include ultraviolet absorbers, as they have been known to cause yellowing with extended exposure. Unlike the present invention, conventional formulations of EVA and PVB include both anti-oxidants and ultraviolet absorbers.
In one embodiment, the encapsulant material comprises a three layer structure. A middle layer is formed of metallocene polyethylene and disposed between two outer layers of ionomer. The layer of metallocene polyethylene can comprise co-polymers of ethylene with butene, hexene, or octene. The ionomer layers can be derived from any direct or grafted ethylene co-polymer of an alpha olefin having the formula Rxe2x80x94CHxe2x95x90CH2, where R is a radical selected from the class consisting of hydrogen and alkyl radicals having from 1 to 8 carbon atoms and alpha, beta-ethylenically unsaturated carboxylic acid having from 3 to 8 carbon atoms.
In another embodiment, the UV light stabilization additive package is incorporated in an encapsulant surrounding a solar cell within a module. The solar cell module comprises at least one solar cell and a transparent encapsulant material disposed adjacent to at least one surface of the solar cell. The encapsulant material comprises at least a polyethylene co-polymer and the ultraviolet light stabilization additive package. A front support layer formed of light transmitting material is disposed adjacent a front surface of the encapsulant material, and a backskin layer is disposed adjacent a rear surface of the encapsulant material. In one embodiment, the encapsulant material comprises a first encapsulant material disposed adjacent a front surface of the solar cell and a second encapsulant layer disposed adjacent a rear surface of the solar cell.
In another embodiment, the ultraviolet light stabilization additive package is included in an encapsulant layer within a laminated transparent member. The laminated transparent member comprises a front support layer, the encapsulant layer, and a rear support layer. All three layers are transparent. The transparent encapsulant layer is disposed adjacent a rear surface of the front support layer. The encapsulant layer comprises a polyethylene co-polymer and the ultraviolet light stabilization additive package. The rear support layer is disposed adjacent a rear surface of the encapsulant layer.
The invention also features a method of manufacturing a solar cell module. According to the method, at least one solar cell is provided, a transparent encapsulant layer is formed and positioned adjacent at least one surface of the solar cell, and the solar cell and the encapsulant material are placed between a transparent front support layer and a backskin layer. The transparent encapsulant layer includes a polyethylene co-polymer and the ultraviolet light stabilization additive package.
In still another embodiment, the invention features a method of manufacturing a laminated transparent member. Two support layers formed of transparent materials are provided, and a transparent encapsulant layer is formed and placed between the support layers to form an assembly. The assembly is laminated. The transparent encapsulant layer includes a polyethylene co-polymer and the ultraviolet light stabilization additive package.